As a label sheet attached to various members and instruments, a pressure-sensitive adhesive sheet having a pressure-sensitive adhesive layer formed by applying a pressure-sensitive adhesive to a substrate sheet has been known.
Such a pressure-sensitive adhesive sheet has been widely used in many industrial fields for print label applications, packaging applications, and the like due to the convenience of being immediately attached to the target product by merely applying pressure. In the production control of automobiles and electronic/electronic products, a pressure-sensitive adhesive sheet (label) on which a barcode is printed is attached to parts, for example.
In the production control of automobiles and electronic/electronic products, the parts may be subjected to a heat treatment. Therefore, the label used is required to exhibit heat resistance.
As such a heat-resistant barcode label, a polyimide film or a polyetherimide film has been used. However, since such a film is expensive, the cost of the resulting barcode label is increased. A polyethylene terephthalate film or a polyethylene film generally used as a substrate for a barcode label is inexpensive, but exhibits poor heat resistance.
In order to solve such a problem, JP-A-2002-275438 proposes a heat-resistant label having a print coating layer on one side of a substrate formed by a polyethylene naphthalate film and having a heat-resistant pressure-sensitive adhesive layer exhibiting an adhesion of 0.5 N/25 mm or more at 150° C. on the other side of the substrate. In JP-A-2002-275438, a rubber-based or acrylic pressure-sensitive adhesive or the like is used as the pressure-sensitive adhesive of the heat-resistant pressure-sensitive adhesive layer.
JP-A-2003-138229 discloses a pressure-sensitive adhesive sheet in which a heat-sensitive pressure-sensitive adhesive layer including a heat-resistant aqueous pressure-sensitive adhesive is formed on at least one side of a substrate sheet. As the heat-resistant pressure-sensitive aqueous adhesive, a known aqueous pressure-sensitive adhesive containing an acrylic polymer emulsion or rubber-based latex as the major component and containing a tackifying resin emulsion is used.
However, the pressure-sensitive adhesives used for the pressure-sensitive adhesive sheets disclosed in the above documents do not necessarily exhibit sufficient heat resistance. Therefore, development of a pressure-sensitive adhesive which exhibits further improved heat resistance and adhesion has been demanded.
A method of causing a vinyl compound such as styrene to undergo living radical polymerization by ultraviolet irradiation using a compound including a photoiniferter group as an iniferter (initiator) has been known (Polymer Bulletin, 7, 197 (1982), Polymer Bulletin, 11, 135 (1984), Macromolecules, 27, 5527 (1994), Macromolecules, 31, 5559 (1998), J. Polym. Sci.; Part A; Polym. Chem., 40, 1885 (2002), Polymer Journal, 34, 10, 736 (2002), and Macromolecules, 36, 2990 (2003)).
The present invention was achieved in view of the above-described situation of the related art. An object of the present invention is to provide a process for producing a polysilsesquioxane graft polymer which may be used as a pressure-sensitive adhesive exhibiting excellent heat resistance and cohesive force, a polysilsesquioxane compound including an iniferter group, and a pressure-sensitive adhesive and a pressure-sensitive adhesive sheet using the resulting polysilsesquioxane graft polymer.